A worryingly large amount of design decisions of the Model M layout were dictated by terminal keyboards and the Selectric typewriter...
To my way of thinking,
not enough of the design decisions followed these two sources.
In 1981,
at the very beginning, they should have designed the IBM PC keyboard to have only 44 typing keys in the main typing area, in the same layout as the original Selectric... with a key having the function of the AltGr key being used, right from the beginning, on the U.S. keyboard, to type {, }, [, ], ~ and `.
That way, the Shift keys, the Enter key, and the Back Space key could have started out in their standard locations, and stayed there.
It shouldn't have taken until the Model M for IBM to say, "Gosh! We've got these 3270 terminals with twelve or twenty-four function keys, and someone might want to emulate them on a PC" and move from ten function keys to twelve. (Of course, the ten function keys on the PC were intended to correspond to a different group of non-programmable special-function keys on those terminals, which was even more confusing...)
Since the Model M has to go through gyrations to simulate an 84-key keyboard from the AT, some keys are useless as buttons for gaming, potentially creating incompatibilities with games written for the PC before the Model M keyboard came out. Switching to Scan Code Set 3 just was not an option. Which shows the importance of getting things right the first time.